CanvasRenderingContext2D.quadraticCurveTo() 是 Canvas 2D API 新增二次贝塞尔曲线路径的方法。它需要 2 个点。第一个点是控制点，第二个点是终点。起始点是当前路径最新的点，当创建二次贝赛尔曲线之前，可以使用 moveTo() 方法进行改变。
语法
void ctx.quadraticCurveTo(cpx, cpy, x, y);参数
cpx
控制点的 x 轴坐标。
cpy
控制点的 y 轴坐标。
x
终点的 x 轴坐标。
y
终点的 y 轴坐标。
示例
quadraticCurveTo 如何工作
这是一段绘制二次贝赛尔曲线的简单的代码片段。控制点是红色，起点和终点是蓝色。
HTML
<canvas id="canvas"></canvas>JavaScript
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
// Quadratic Bézier curve
ctx.beginPath();
ctx.moveTo(50, 20);
ctx.quadraticCurveTo(230, 30, 50, 100);
ctx.stroke();
// Start and end points
ctx.fillStyle = 'blue';
ctx.beginPath();
ctx.arc(50, 20, 5, 0, 2 * Math.PI); // Start point
ctx.arc(50, 100, 5, 0, 2 * Math.PI); // End point
ctx.fill();
// Control point
ctx.fillStyle = 'red';
ctx.beginPath();
ctx.arc(230, 30, 5, 0, 2 * Math.PI);
ctx.fill();
看DEMO演示：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>CanvasRenderingContext2D.quadraticCurveTo() | Web176教程www.web176.com</title>
<meta name="robots" content="noindex, nofollow">
<style>
body {
padding: 0;
margin: 0;
}
svg:not(:root) {
display: block;
}
.playable-code {
background-color: #f4f7f8;
border: none;
border-left: 6px solid #558abb;
border-width: medium medium medium 6px;
color: #4d4e53;
height: 100px;
width: 90%%;
padding: 10px 10px 0;
}
.playable-canvas {
border: 1px solid #4d4e53;
border-radius: 2px;
}
.playable-buttons {
text-align: right;
width: 90%%;
padding: 5px 10px 5px 26px;
}
</style>
</head>
<body>
<canvas id="canvas"></canvas>
<script>
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
// Quadratic Bézier curve
ctx.beginPath();
ctx.moveTo(50, 20);
ctx.quadraticCurveTo(230, 30, 50, 100);
ctx.stroke();
// Start and end points
ctx.fillStyle = 'blue';
ctx.beginPath();
ctx.arc(50, 20, 5, 0, 2 * Math.PI); // Start point
ctx.arc(50, 100, 5, 0, 2 * Math.PI); // End point
ctx.fill();
// Control point
ctx.fillStyle = 'red';
ctx.beginPath();
ctx.arc(230, 30, 5, 0, 2 * Math.PI);
ctx.fill();
// const canvas = document.getElementById('canvas');
// const ctx = canvas.getContext('2d');
// Quadratic Bézier curve
ctx.beginPath();
ctx.moveTo(50, 20);
ctx.quadraticCurveTo(230, 30, 50, 100);
ctx.stroke();
// Start and end points
ctx.fillStyle = 'blue';
ctx.beginPath();
ctx.arc(50, 20, 5, 0, 2 * Math.PI); // Start point
ctx.arc(50, 100, 5, 0, 2 * Math.PI); // End point
ctx.fill();
// Control point
ctx.fillStyle = 'red';
ctx.beginPath();
ctx.arc(230, 30, 5, 0, 2 * Math.PI);
ctx.fill();
</script>
</body>
</html>
简单的二次曲线
此示例使用quadraticCurveTo()绘制了简单的二次 Bézier 曲线。
HTML
<canvas id="canvas"></canvas>JavaScript
曲线从moveTo()指定的点开始：(20, 110)。控制点位于 (230, 150)。曲线在 (250, 20) 处结束。
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(20, 110);
ctx.quadraticCurveTo(230, 150, 250, 20);
ctx.stroke();
结果：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>CanvasRenderingContext2D.quadraticCurveTo() | Web176教程www.web176.com</title>
<meta name="robots" content="noindex, nofollow">
<style>
body {
padding: 0;
margin: 0;
}
svg:not(:root) {
display: block;
}
.playable-code {
background-color: #f4f7f8;
border: none;
border-left: 6px solid #558abb;
border-width: medium medium medium 6px;
color: #4d4e53;
height: 100px;
width: 90%%;
padding: 10px 10px 0;
}
.playable-canvas {
border: 1px solid #4d4e53;
border-radius: 2px;
}
.playable-buttons {
text-align: right;
width: 90%%;
padding: 5px 10px 5px 26px;
}
</style>
</head>
<body>
<canvas id="canvas"></canvas>
<canvas id="canvas" width="400" height="200" class="playable-canvas"></canvas>
<div class="playable-buttons">
<input id="edit" type="button" value="Edit" />
<input id="reset" type="button" value="Reset" />
</div>
<textarea id="code" class="playable-code">
ctx.beginPath();
ctx.moveTo(50,20);
ctx.quadraticCurveTo(230, 30, 50, 100);
ctx.stroke();</textarea>
<script>
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(20, 110);
ctx.quadraticCurveTo(230, 150, 250, 20);
ctx.stroke();
var textarea = document.getElementById("code");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;
function drawCanvas() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
eval(textarea.value);
}
reset.addEventListener("click", function() {
textarea.value = code;
drawCanvas();
});
edit.addEventListener("click", function() {
textarea.focus();
})
textarea.addEventListener("input", drawCanvas);
window.addEventListener("load", drawCanvas);
</script>
</body>
</html>
作者：terry，如若转载，请注明出处：https://www.web176.com/canvas_api/7755.html
